Marine and land-based industrial (M & I) gas turbine engines are frequently derived from engines designed for aircraft because it can be cost effective to develop an M & I engine by modifying an existing aircraft gas turbine engine in the desired power class. One M & I engine application provides output shaft horsepower for powering an electrical generator at a synchronous speed, such as 3000 rpm or 3600 rpm for generating electricity at 50 Hz or 60 Hz. To keep development costs and kilowatt-hour costs low, M & I engine designers typically use a parent aircraft engine and make as few changes in the parent engine as needed for obtaining the desired land-based M & I engine.
One type of M & I engine used for powering an electrical generator can include two rotors. A first low pressure rotor system can include a power turbine which powers a booster compressor through a first low pressure shaft, and a load, such as an electrical generator, through an output shaft. Power turbine horsepower not required to drive the booster compressor is available as output shaft horsepower to drive the electrical generator. The booster compressor, power turbine, and output shaft are mechanically coupled and rotate together. A second core engine high pressure rotor system includes a conventional high pressure compressor (HPC) driven by a conventional high pressure turbine (HPT) through a second high pressure rotor shaft.
In the parent aircraft engine a reduction in power level setting or fuel flow to the core engine would require a corresponding reduction in speed of the power turbine and booster compressor. This reduction in speed would be necessary to match the flow delivered by the booster compressor to the flow required by the the core engine at the reduced power level. However, in the M & I derivative engine the power turbine and booster compressor must rotate at the constant synchronous speed of the electrical generator at both high and low power settings of the core engine, and regardless of the horsepower required at the output shaft by the electrical generator. The parent engine was initially designed for providing substantial horsepower from the power turbine at the synchronous speed for powering the fan in the parent engine. Thus, at low core power settings the booster compressor in the industrial derivative engine will tend to deliver more airflow than is required to the core engine, which can result in booster compressor stall. This problem can occur, for instance, during lock-on or lock-off of the generator from the electric power grid, or during an emergency unfueled shut-down of the engine.
Single panel variable inlet guide vanes (VIGVS) postioned at the inlet of the booster compressor can be partially closed to reduce booster flow to the compressor and to reduce power turbine horsepower. In addition, variable bleed valves (VBVS) can be used with booster VIGVs to further reduce the amount of booster flow entering the core engine and the power turbine horsepower. Accordingly, the parent aircraft engine could be further modified by replacing the original VBVs with larger VBVs for bleeding additional compressed air from the booster compressor, and the VIGVs could also be modified for closing even further the booster compressor inlet. However, larger VBV's are generally undesirable since they require additional structural changes to the parent engine, and larger VBVs require larger openings that can reduce the stiffness and load bearing capability of load carrying engine structures in which they are formed. In one exemplary engine application, the required flow area of the VBVs in the M & I engine would have to be increased twice as large as the original flow area of the VBVs in the parent aircraft engine for reducing the output shaft horsepower to a substantially zero value for allowing lock-on and lock-off of the generator to the electrical grid. In addition, further closure of conventional single panel VIGVs can result in undesirable pressure and temperature distortions in the compressed airflow channeled to the core engine. Such distortion can result in core compressor stall and possible damage to the core engine.
Thus, engineers and scientists continue to seek improved modifications of parent aircraft engines to obtain industrial gas turbine derivative engines.